The desired valuable minerals are separated from the ores in the mining industry by flotation. This is effected in flotation cells of continuous flow type, in which air is conducted into vigorously mixed slurry of ground ore and water. Due to chemical preprocessing, the grains of the valuable mineral tend to adhere selectively on surfaces of air bubbles, to be lifted with these from the slurry to the froth layer above its surface. At the same time, also other mineral grains and mixed (locked) grains of a weaker tendency to float rise to this layer, and return from froth to slurry takes place as well. The froth flows continuously over the cell edge down into a launder producing the concentrate of the cell.
The final concentrate of an industrial flotation circuit consists of concentrates of individual flotation cells, which usually have been cleaned by refloating them, often in several states. The content of the valuable mineral in the concentrate of the cell is, together with the recovery of the valuable mineral, the most important factor on which the economic value of its concentrate depends. Therefore the quality of the final concentrate and, at long intervals, also that of concentrates of the individual cells is controlled by taking samples and analyzing them in laboratory. The most important one of the instruments for immediate measurement of slurries in a flotation plant is the X-ray fluorescence analyzer which mostly analyzes metal contents of solids contained in sample streams separated from slurries. For its high price, this device however does not apply to analysis of the concentrate of a single cell, but instead of that analyzes joint samples of cell combinations or complete flotation circuits. The need for development of an instrument for on-line analysis of operation of single flotation cells or for that of material processed by them is therefore high. For this reason, an attention has recently been paid also to such measurements which relate to the flotation froth.
The appearance of the froth describes sensitively the operational state of the froth layer and even that of the whole cell, because all the material passing it and contained by it arrives to it through the slurry space of the cell. The surface of the froth is visible and the process controller traditionally inspects it by naked eye, in order to observe qualitatively its general outlook and specific features and to then base his manual control actions on his observations and conclusions. Thus he may describe the froth e.g. one of big bubbles, porridge-type, watery, dry, stiff etc., in addition to characterization of its color.
The quantitative, instrumental evaluation of the froth has become possible, as the combination of the video camera and the computer, connected to it for the analysis of the electric image signal, has become available. Since then, several research groups have directed their work to processing of pictures taken of the flotation froth, either in order to determine structures of froths from black/white pictures (e.g. Moolman D. W. and al. in Int. J. Miner. Process. 43(1995), 193-208) or colors of froths from multi-, i.e. usually three-color pictures (e.g. Oestreich J. M. and al. in Minerals Engineering 8(1995), 31-39). Apparatuses with their software used for these aims have since then been subjected to commercialization. It is typical to said studies and apparatuses to observe a rectangular, rather large part of the industrial cell""s froth surface, whose horizontal area is typically considerably larger than one square meter, and to process the sampled surface of said type as a representative sample of the cell""s froth surface.
The conventional semiconductor matrix video camera device has been used in the stated studies. The U.S. Pat. No 4,831,641 states, for its part, the analysis of flowing suspension in the mineral refining industry and, more particularly, the identification of solid particles in a flowing process fluid, without distinguishing the semiconductor matrix and semiconductor line array cameras from each other. It does not mention the flotation froth, and with suspensions in the stated industry one usually means two-phase solid/liquid suspensions and not the three-phase flotation froth. The illumination of the object is not presented at all in the stated patent.
An individual bubble can, if the camera and light source are located above the cell, be distinguished by means of light, which is reflected strongly back by its top area. This small, bright spot is in such a case surrounded by a darker zone. Depending on the illumination, the darkest regions may lie at the border of two bubbles, but the bottom of the valley separating the bubbles appears often also bright, because of the light it reflects, or is manifested by a stepwise change of the degree of darkness. Determination of the structural parameters of the froth, such as the mean bubble size and the form, density and size distribution of the bubbles can, further on, be based on the borderlines. The speed of the froth""s movement is, for its part, determined by comparing successive pictures with each other. It is also customary to determine the brightness distribution of the imaged area and to present it in the form of a histogram. Features of the structure can also be determined by means of other statistical methods, on the basis of the frequency of appearance of image elements of different degrees of darkness.xe2x80x94The deterministically and statistically determinable features stated above are examples of quantities which have been determined by image analysis and presented in the literature, and which are generally characterized by a considerably large scatter.
By means of a color video or color television camera one obtains, of the imaged field of the same type, a red, green and blue (RGB) signal, which signal set or the composite signal of standard form corresponding to it can be processed as such or transformed to other code form before processing. Determination of the color or spectrum of the froth suffers from large differences of intensity of the light reflected specularly or diffusely by the froth. Therefore e.g. too high signal elements have to be removed before processing. The color observed depends on mineral composition of the froth, but the determination of this dependence meets difficulties in practice which, in addition to the said differences of intensity, is due to the rather small differences of color of the colored metal minerals and to other, generally black/gray/white minerals present and to variation of their concentrations. Determination of both the structure and color is affected by the inhomogeneity of the quantities observable in the fields of view of the said camera instruments. This has not been taken into account in the studies reported earlier and has at least not influenced their methodologies or hardware technologies; it shall be reverted to lower down.
For the stated determinations described in the literature, one has used previously known computational algorithms or mathematical methods, which have been programmed to the form required by numerical computation in accordance with the aim of use described, or are obtainable from software libraries (See e.g. Niemi A. J. and al. in Int. J. Miner. Process. 51(1997), 51-65 and several of its reference publications). Results of the determinations can be exploited in flotation control, but because their dependence on the input quantities of flotation is generally not accurately nor unambiguously known, the statements on control and regulation have remained on the stage of draft in the literature.
In an ideally operating flotation cell, the entering air is distributed symmetrically around its axis in the horizontal plane, an the bubbles are distributed homogeneously, still as they reach the lower interface of the froth layer. The froth leaves the cell, which typically has the form of a rectangular parallelepiped, over one of its edges, or sometimes over its two opposite, parallel edges. Thus the liquid and solids which are carried along by the bubbles which rise to the froth layer in the rear part (correspondingly sometimes in the middle part) of the cell spend the longest time for their travel to the overflow edge and out of the cell. The amount of the stated components and of these, especially that of the others than the floatable primary mineral certainly decreases during the travel, as the bubbles break and join to each other, and as their grains either adhere to the bubbles aside or below them, displacing grains adhered more weakly to these, or flow between the bubbles down to the slurry space. That part of the material lifted in the rear part which stays in the surface layer of the froth moves, at first slowly and then, at an increasing speed, toward the overflow edge. The acceleration is caused by the new material rising everywhere to the froth layer which, despite of the selective return of the solids, gives a continuous impulse directed toward the free edge of the froth. Thus the surface is continuously reached by material lifted to the froth nearer to the overflow edge whose residence in the froth remains shorter and liberation from gangue en components less.xe2x80x94Theoretical models of the froth layer have been derived (e.g. Moys, M. H. in Frothing in Flotation (Editor J. S. Laskowski), Gordon and Breach, UK 1989, 203-228), but they have not enabled one to make practical conclusions on the mineral concentrations of the froth surface.
As a result of the process described, the mineral composition of the froth surface changes at the transfer toward the overflow edge. This change and the continuous growth of the transfer rate in the same direction imply an inclination toward inhomogeneity, also in the structure of the froth. The inhomogeneity of the industrial flotation froth has accordingly been stated in the literature of the branch (Laplante A. R. and al. in Min. Proc. Extr. Met. Rev. 5(1989), 147-168). More lately, Niemi A. J. and al. (Int. J. Miner. Process. 51(1997), 51-65) have, in their study of apatite flotation cells, observed that the relatively light color of the froth in the rear part of the cell corresponds to a higher apatite content than that of the froth in the neighborhood of the overflow edge, the color of the latter being clearly affected by the red-brown mica present as a gangue. A video camera imaging a rather large area of the form of a rectangle has been used in the latter, experimental study in which the analysis of partial areas of the images transmitted by the camera has produced the stated result.
One may further conclude that the analysis of the information contained by a relatively large surface area as an entity, neglecting the differences of color and structure between its different parts, delivers only average results. With regard to the concentrate produced by the cell, they have to be considered as rough approximations, especially paying attention to the fact that the part of the image area which is closest to the overflow edge is, with regard to the concentrate being formed, much more significant than the other parts or averages of the imaged area. The observation is also influenced by the inhomogeneity of illumination of the object which is the greater the larger the area of the froth surface being measured and which correspondingly distorts the image being formed on the detector.
On the basis of the stated studies and the character of the process, it is obvious that, in the surface layer of the flotation froth, the age distributions and transfer rates of the liquid phase and of the different phases of solids and, further on, the mineral compositions of the solids change, as one moves, within the surface of the froth, from the rear part (middle part) toward the overflow edge. It is also obvious that these changes have an influence on the structure and the visually observable properties of the surface layer. The age of a material element means here the time, which has been spent since its transfer to the froth layer from the slurry space.
On the other hand, there are no physical reasons to assume that changes would be present in the direction which is perpendicular to the direction of the stated movement, i.e. in that of travel from one side of the cell to the other parallelly to the overflow edge, excluding the effect which the side walls of the cell may have in their immediate neighborhood on the movement and structure of the froth. The central feature of the new method is accordingly the acquisition of representative image information of the froth layer in such a manner that the observation and the analysis of the result of observation are directed to a narrow strip of the surface, which strip is parallel to the overflow edge. The length of a strip of this kind, within which the quality of the froth is essentially even, may be equal to the breadth of the cell or less than this, e.g. in the presence of said side wall effect or for other reason, such as one related to the technology of the measuring device.
Even when the flotation process operates in a stationary state, the outcome of a momentary measurement and the property determined on the basis thereof differ from the stationary value because of the process noise. E.g. the diameter of even one, big bubble may be several percents of the length of the measured strip, and therefore the structure, brightness, color and other properties of the froth have to be determined as averaged and distributed quantities over the strip, usually by means of several, successive observations and gliding determinations. The quantities obtained in such a manner describe the properties of the froth at each chosen location better than the quantities determined over a larger surface, which has been assumed homogeneous but is inhomogeneous in reality.
Location of the strip to be observed depends on the primary aim of the observation and analysis. With regard to the froth structure, it is best placed before the overflow edge, at a location where the speed differences caused by the overflow do not yet deform the bubbles. The data obtainable near the overflow edge and after this describes, for its part, better the final concentrate of the cell, especially with regard to the color and therefore also to the mineral concentration. On the other hand, the observation and measurement before the overflow edge can in principle, at no change of the other input quantities than the mineral concentration of the cell feed, be calibrated to indicate the mineral concentration as well.
Homogeneous illumination of the froth strip being observed can require construction of reflective surfaces and that of screens for elimination of external light, in accordance with the aim of use and the local conditions of use, but the In device system for stated determinations may, in other respects, consist, for its main part, of a combination of commercially available devices. Some of such devices are AC and DC lamps of an appropriate power and emission spectrum, optical filters, semiconductor line array cameras (in special cases matrix cameras) with conventional lens optics which take black/white and color pictures at adjustable or fixed intervals, digital, primarily micro computers, devices for transfer of information between said devices and output devices for results of measurements and analyses.
The device system for carrying out the observation and analysis according to the method described may consist of e.g. the apparatus according to U.S. Pat. No 4,831,641 comprising a linear photodiode array detector. In such a system, a decreased image of the aimed froth strip and of its immediate environment is formed by means of conventional, spherical optics in such a manner that the strip is imaged within the detector""s area, i.e. an optics of a suitable focal length and its distance from the froth are chosen so, that the imaging takes place in the manner described. The dimensions of the detector determine, at the same time, the breadth of the froth strip being observed, and the number of its elements the resolution of the observation in the direction of the strip. The scanning rate of the detector is then chosen suitably so, that each element of the surface will be read approximately once, as the froth moves at its average speed. In addition to the detectors which consist of one semiconductor line array, also such integrated detectors arc available which comprise several, parallelly located line array detectors. In them, the signals of parallel elements are added to each other and they deliver only one, serial output signal; the line array detector stated earlier is, above and in the following, considered to comprise also such detectors.xe2x80x94Line scan cameras are obtainable for such uses as industrial products which sufficiently meet the requirements set to the optics, resolution of the observation and reading frequency of the detector. In addition to the one-color cameras, RGB and other color cameras are obtainable, in which the colors of the optical image signal either are separated by filters or the image signal is divided by e.g. a prism to its components, which are then guided to different semiconductor line array detectors. The optical signal range is in the following considered to cover the range of the electromagnetic, both visible and invisible radiation, such as that of the infrared and ultraviolet light to the extent to which the lens optics and semiconductor detectors are able to operate.
The electric, discrete signal corresponding to the optical image signal is read from the detector elements in the form of series of pulses, and each signal element is converted to a digital number, which is proportional to the amplitude of the signal element and therefore to the level of grayness of the image element. The reading and processing of the data and transfer of data for processing into the central unit or for storage into the memory of the computer thus proceed practically e.g. in the manner described in the stated patent. Alternatively, the information measured can also be transferred in the form of a continuous analog signal which is discretized and converted to digital data in the interface unit of the computer.
Transfer of data into the central unit of the computer and storage into its memory are programmed for implementation of such previously known methods which have been used for analysis and interpretation of information transmitted by the line array camera in the monitoring of fixed, mechanically movable pieces, like in the classification of rocks moved on a conveyor belt in the mining industry or in search and observation of surface defects of metal plates in connection with the rolling. The processing of data is programmed for implementation of the numerical methods, which are known from the analysis of a large froth surface on the basis of the image transmitted by a matrix camera, after they have been reduced to processing of one-dimensional data, or using them in their two-dimensional form, in the manner to be described in the following. Thus the grayness histogram of the froth strip is obtained by arranging the measured data according to their degree of grayness and the function describing the texture of the strip from variation of the grayness by means of e.g. the Fourier transformation, both of them as average functions of a number of measurements and analyses. Corresponding to this, the breadths of the bubbles and the distribution of the breadth are obtained as quantities, which are determined deterministically.
A two-dimensional representation of the froth at the location of the strip is constructed, for determination of forms of the bubbles and for that of e.g. two-dimensional, statistical quantities, by joining the successive strip signals to each other. A picture of this kind represents the froth in the stationary state better than any momentary observation transmitted by the naked eye or matrix camera, or an average picture derived from such observations. The constructed, two dimensional picture can then be processed by means of previously known methods, which have been used for the processing of froth pictures taken with the matrix camera. It can, further on, be displayed on monitor as a picture representing the strip under observation, for visual inspection and detection of properties of the froth and for that of their changes.xe2x80x94However, if the speed of the froth is subject to variations, it has in such a case to be determined separately and taken into account at the joining of the strip signal to the previous one. Here one may take a benefit of e.g. the fact that such bubbles which are small but have a breadth corresponding to at least several image elements are, in most mineral froths, circular as seen from above. Under such conditions one may determine, at times and using as may be needed a faster scanning frequency of the semiconductor line array detector, the time interval during which the front or rear end of each chosen bubble passes the field of view or its edges. The momentary speed of the froth in the principal direction of flow is calculated on the basis of the stated time interval and the measured breadth of the bubble, which is equal to its diameter.